1. Field of the Invention
The present invention pertains to thermoplastic polyurethanes. More particularly, the present invention pertains to thermoplastic polyurethanes exhibiting reduced cycle time in molding operations.
2. Description of the Related Art
Commercial applications of thermoplastic polyurethanes (TPU) continue to grow at a rapid pace. Unlike their thermoset relatives, TPU can be processed in a manner similar to other thermoplastics in operations such as extrusion, injection molding, wire coating, etc. Useful TPU for such operations usually have a hardness after of at least 80 Shore A. The TPU will typically have a hardness less than or equal to 74 Shore D.
The productivity associated with injection-moldable thermoplastics depends to a great extent on the cycle time associated with the particular molding process. In general, the thermoplastic utilized must exhibit sufficient "green strength" to be demolded, to develop a level of properties sufficient to maintain shape and structural integrity upon demolding. The development of this "green strength" determines length of the solidification portion of the overall molding process.
To control the cycle time, numerous variables in the molding process may be adjusted, however in the final analysis, the amount the cycle time may ultimately be decreased is dependent on the physical properties desired of thermoplastic itself. In polyurethanes, for example, the properties of the TPU may be altered by varying the nature of the diol and diisocyanate which have been reacted to form the TPU. Such methods of varying TPU properties are by now well established in the art. Given a particular TPU, however, physical properties such as melting point, melt viscosity and other properties important to the molding process are fixed.
Addition of plasticizers to TPU may aid in increasing their tensile elongation and may also result in lower melting point and melt viscosity. However the use of plasticizers may also decrease the rate of crystallization, and thus the positive effect on cycle time expected due to lower processing temperatures and/or pressures is negated by the longer solidification time. Furthermore, tensile strength and modulus may be lowered by addition of plasticizer. Finally, plasticizer may bleed out of the finished product, particularly at high ambient temperatures.
Addition of crystallizing agents would be expected to decrease cycle time, and indeed this is frequently the case. However, many crystallization promoters prohibit the final article from being transparent or in many cases even translucent. Moreover, many of such crystallization promoters or nucleating agents decrease certain desirable physical properties such as low temperature impact resistance. Crystallizing agents which are small enough in size or are soluble in the TPU such that transparent articles may be obtained are expensive, and only limited amounts can be utilized.
It would be desirable to utilize an additive which when added to TPU would decrease cycle time without significantly decreasing physical properties such as hardness, tensile strength, modulus, and tensile elongation, or the use of which may actually increase such physical properties.